Preparation of silicon nitride powder

Abstract

Silicon nitride powder may be prepared by subjecting a composite comprising at least a monolayer of a carbonaceous pyropolymer possessing recurring units containing at least carbon and hydrogen atoms on the surface of a silica support to the action of nitrogen-containing atmospheres at nitriding conditions to form silicon nitride.

We claim as our invention: 
     
       1. A process for the production of silicon nitride powder which comprises treating a silica support having a surface area in the range of from about 1 to about 500 m 2  /g by impregnating said support with an organic pyropolymer precursor, pyrolyzing said treated silica support at a temperature in the range of from about 400 to about 1200° C. and at a pressure in the range of from about atmospheric to about 100 atmospheres to form a carbonaceous pyropolymer possessing recurring units containing at least carbon and hydrogen atoms composited on the surface of said silica support, subjecting said support to nitriding conditions including a nitrogen-containing atmosphere which comprises a mixture of nitrogen and hydrogen to yield silicon nitride powder, and recovering the resultant silicon nitride powder. 
     
     
       2. The process as set forth in claim 1 in which said nitriding conditions include a temperature in the range of from about 1300° to about 1500° C. and a pressure in the range of from about 0.5 to about 25 atmospheres. 
     
     
       3. The process as set forth in claim 1 in which said hydrogen is present in said mixture in a range of from about 1% to about 80%. 
     
     
       4. The process as set forth in claim 1 in which said nitrogen-containing atmosphere further comprises ammonia. 
     
     
       5. The process as set forth in claim 1 in which the mole ratio of carbon to silica in said compound is in a range of from about 2 to about 15. 
     
     
       6. The process as set forth in claim 1 in which said silica support comprises fumed silica powder. 
     
     
       7. The process as set forth in claim 1 in which said silicon nitride powder contains from about 87% to about 100% by weight of alpha-silicon nitride. 
     
     
       8. The process as set forth in claim 1 in which said silicon nitride powder contains from about 92% to about 98% by weight of crystalline silicon nitride. 
     
     
       9. The process as set forth in claim 1 in which said organic pyropolymer precursor comprises an organic monomer which is capable of undergoing polymerization prior to pyrolyzation thereof. 
     
     
       10. The process as set forth in claim 9 in which said organic monomer is polymerized at a temperature in the range of from about ambient to about 250° C. and a pressure in the range of from about atmospheric to about 100 atmospheres. 
     
     
       11. The process as set forth in claim 1 in which said pyrolyzation is effected in an inert or reducing atmosphere. 
     
     
       12. The process as set forth in claim 11 in which said inert or reducing atmosphere is provided for by the presence of a gas selected from the group consisting of nitrogen, helium, argon and hydrogen. 
     
     
       13. The process as set forth in claim 1 in which said organic pyropolymer precursor comprises toluene. 
     
     
       14. The process as set forth in claim 1 in which said organic pyropolymer precursor comprises benzene. 
     
     
       15. The process as set forth in claim 1 in which said organic pyropolymer precursor comprises cyclohexane. 
     
     
       16. The process as set forth in claim 9 in which said organic monomer comprises styrene. 
     
     
       17. The process as set forth in claim 9 in which said organic monomer comprises divinylbenzene.